Add support for drag images and drop descriptions

src/System.Windows.Forms.Primitives/src/Interop/Interop.CLSID.cs

@@ -14,5 +14,8 @@ internal static class CLSID
 
         // DC1C5A9C-E88A-4DDE-A5A1-60F82A20AEF7
         internal static Guid FileOpenDialog = new Guid(0xDC1C5A9C, 0xE88A, 0x4DDE, 0xA5, 0xA1, 0x60, 0xF8, 0x2A, 0x20, 0xAE, 0xF7);
+
+        // 4657278A-411B-11D2-839A-00C04FD918D0
+        internal static Guid DragDropHelper = new Guid(0x4657278A, 0x411B, 0x11D2, 0x83, 0x9A, 0x0, 0xC0, 0x4F, 0xD9, 0x18, 0xD0);
     }
 }

src/System.Windows.Forms.Primitives/src/Interop/Shell32/Interop.DROPDESCRIPTION.cs

@@ -0,0 +1,47 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System.Runtime.InteropServices;
+
+internal partial class Interop
+{
+    internal static partial class Shell32
+    {
+        [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
+        public unsafe struct DROPDESCRIPTION
+        {
+            private DROPIMAGETYPE _type;
+            private fixed char _szMessage[Kernel32.MAX_PATH];
+            private fixed char _szInsert[Kernel32.MAX_PATH];
+
+            private Span<char> szMessage
+            {
+                get { fixed (char* c = _szMessage) { return new Span<char>(c, Kernel32.MAX_PATH); } }
+            }
+
+            private Span<char> szInsert
+            {
+                get { fixed (char* c = _szInsert) { return new Span<char>(c, Kernel32.MAX_PATH); } }
+            }
+
+            public DROPIMAGETYPE Type
+            {
+                get => _type;
+                set => _type = value;
+            }
+
+            public ReadOnlySpan<char> Message
+            {
+                get => szMessage.SliceAtFirstNull();
+                set => SpanHelpers.CopyAndTerminate(value, szMessage);
+            }
+
+            public ReadOnlySpan<char> Insert
+            {
+                get => szInsert.SliceAtFirstNull();
+                set => SpanHelpers.CopyAndTerminate(value, szInsert);
+            }
+        }
+    }
+}

src/System.Windows.Forms.Primitives/src/Interop/Shell32/Interop.DROPIMAGETYPE.cs

@@ -0,0 +1,21 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+internal partial class Interop
+{
+    internal static partial class Shell32
+    {
+        public enum DROPIMAGETYPE
+        {
+            DROPIMAGE_INVALID = -1,
+            DROPIMAGE_NONE = 0,
+            DROPIMAGE_COPY = 1,
+            DROPIMAGE_MOVE = 2,
+            DROPIMAGE_LINK = 4,
+            DROPIMAGE_LABEL = 6,
+            DROPIMAGE_WARNING = 7,
+            DROPIMAGE_NOIMAGE = 8
+        }
+    }
+}

src/System.Windows.Forms.Primitives/src/Interop/Shell32/Interop.IDropTargetHelper.cs

@@ -0,0 +1,46 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System.Drawing;
+using System.Runtime.InteropServices;
+using IComDataObject = System.Runtime.InteropServices.ComTypes.IDataObject;
+
+internal static partial class Interop
+{
+    internal static partial class Shell32
+    {
+        [ComImport]
+        [Guid("4657278B-411B-11D2-839A-00C04FD918D0")]
+        [InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
+        public interface IDropTargetHelper
+        {
+            [PreserveSig]
+            HRESULT DragEnter(
+                IntPtr hwndTarget,
+                [MarshalAs(UnmanagedType.Interface)]
+                IComDataObject pDataObj,
+                ref Point ppt,
+                uint dwEffect);
+
+            [PreserveSig]
+            HRESULT DragLeave();
+
+            [PreserveSig]
+            HRESULT DragOver(
+                ref Point ppt,
+                uint dwEffect);
+
+            [PreserveSig]
+            HRESULT Drop(
+                [MarshalAs(UnmanagedType.Interface)]
+                IComDataObject pDataObj,
+                ref Point ppt,
+                uint dwEffect);
+
+            [PreserveSig]
+            HRESULT Show(
+                BOOL fShow);
+        }
+    }
+}

src/System.Windows.Forms/src/PublicAPI.Unshipped.txt

@@ -1,6 +1,21 @@
-~override System.Windows.Forms.SplitContainer.CreateAccessibilityInstance() -> System.Windows.Forms.AccessibleObject
 ~override System.Windows.Forms.PrintPreviewControl.CreateAccessibilityInstance() -> System.Windows.Forms.AccessibleObject
 ~override System.Windows.Forms.DateTimePicker.DateTimePickerAccessibleObject.Name.get -> string
 ~override System.Windows.Forms.DateTimePicker.OnGotFocus(System.EventArgs e) -> void
 override System.Windows.Forms.DateTimePicker.DateTimePickerAccessibleObject.DoDefaultAction() -> void
 ~override System.Windows.Forms.DateTimePicker.DateTimePickerAccessibleObject.DefaultAction.get -> string
+~override System.Windows.Forms.SplitContainer.CreateAccessibilityInstance() -> System.Windows.Forms.AccessibleObject
+System.Windows.Forms.DragEventArgs.DropIcon.get -> System.Windows.Forms.DropIconType
+System.Windows.Forms.DragEventArgs.DropIcon.set -> void
+System.Windows.Forms.DragEventArgs.Insert.get -> string!
+System.Windows.Forms.DragEventArgs.Insert.set -> void
+System.Windows.Forms.DragEventArgs.Message.get -> string!
+System.Windows.Forms.DragEventArgs.Message.set -> void
+System.Windows.Forms.DropIconType
+System.Windows.Forms.DropIconType.Copy = 1 -> System.Windows.Forms.DropIconType
+System.Windows.Forms.DropIconType.Default = -1 -> System.Windows.Forms.DropIconType
+System.Windows.Forms.DropIconType.Label = 6 -> System.Windows.Forms.DropIconType
+System.Windows.Forms.DropIconType.Link = 4 -> System.Windows.Forms.DropIconType
+System.Windows.Forms.DropIconType.Move = 2 -> System.Windows.Forms.DropIconType
+System.Windows.Forms.DropIconType.NoDropIcon = 8 -> System.Windows.Forms.DropIconType
+System.Windows.Forms.DropIconType.None = 0 -> System.Windows.Forms.DropIconType
+System.Windows.Forms.DropIconType.Warning = 7 -> System.Windows.Forms.DropIconType